A common silicon carbon (SiC) based power semiconductor rectifier is shown in FIG. 1 in cross section. It comprises a SiC wafer including a substrate layer 1, which is made of highly doped n-type SiC, and a drift layer 2, which is made of low-doped n-type SiC formed on the substrate layer 1. The SiC wafer has a first main side 3 and a second main side 4 parallel to the first main side 3. The first main side 3 of SiC wafer, which is the anode side of the device, is covered with a first metal contact layer 5 that forms a Schottky contact with the drift layer 2. On the second main side 4, which is the cathode side of the device, there is formed a second metal contact layer 6. Typically the drift layer 2 is grown epitaxially on a highly doped n-type SiC substrate wafer used as the substrate layer 1.
Depending on the electrical polarity of the voltage between anode and cathode, the Schottky contact either blocks current flow or allows the passage of majority carriers (which are electrons in n-doped semiconductor material). These two modes correspond with the blocking and on-state operation of the power semiconductor rectifier.
In FIGS. 2a and 2b there is shown a SiC based junction barrier Schottky (JBS) rectifier which is another common SiC based power semiconductor rectifier. A JBS rectifier is a hybrid power device, which combines a Schottky and a pin diode structure in one device, making use of the advantages of both structures. It has a low on-state resistance and a high blocking capability. Silicon carbide (SiC) based JBS rectifiers are candidates to replace silicon (Si) based pin diodes for high blocking voltages. SiC material properties allow devices with higher voltage rating and higher operating temperatures compared to Si.
FIG. 2a shows a vertical cross-section vertical to a first main side 3 of the device, whereas FIG. 2b shows a horizontal cross section along line AA′ in FIG. 2a and parallel to the first main side 3. Like the power semiconductor rectifier shown in FIG. 1, the JBS rectifier shown in FIGS. 2a and 2b comprises a SiC wafer including a substrate layer 1, which is made of highly doped n-type SiC, and a drift layer 2, which is made of low-doped n-type SiC formed on the substrate layer 1. The SiC wafer has a first main side 3, which is the first main side of the device, and a second main side 4 parallel to the first main side 3. Adjacent to the surface of the drift layer 2, on the first main side 3 opposite to the substrate layer 1, there are formed a plurality of p-type emitter regions 7. The first main side 3 of the SiC wafer, which is the anode side of the device, is covered with the first metal contact layer 5 that forms a Schottky contact in places where the first metal contact layer 5 contacts the n-type drift layer 2 and that forms an ohmic contact with the p-type emitter regions 7 in places where the first metal contact layer 5 contacts the p-type emitter regions 7. Throughout this specifications the term ohmic contact refers to a non-rectifying junction between two materials, which has linear current-voltage characteristics. In contrast thereto the term Schottky contact refers throughout this specifications to a rectifying junction between a semiconductor and a metal, which has non-linear current-voltage characteristics.
The blocking capability of the above known power semiconductor rectifiers is mainly given by the thickness and doping density of the n-doped drift layer 2. However, as a result of the nature of the Schottky contact, image force lowering at elevated electric field levels at high blocking voltages causes the barrier for electrons to shrink. The power semiconductor rectifier shown in FIG. 1, which is a pure Schottky barrier diode without p-doped emitter regions 7, will be prone to increasing levels of leakage currents at high reverse bias. The comparatively large number of carriers will entail intensified electron-hole pair generation during impact ionization. As a result, the power semiconductor rectifier shown in FIG. 1 exhibits a relatively high leakage current and a low breakdown voltage. In a the JBS rectifier shown in FIGS. 2a and 2b, the p-type emitter regions 7 help to improve this situation. Under reverse bias, a depletion layer develops across the pn-junctions between the p-type emitter regions 7 and the n-type drift layer 2 in the same way as it does in a pin diode, The individual depletion zones around the p-doped emitter regions 7 may eventually connect with each other and close in between two adjacent emitter regions 7 below the Schottky contact. In this way the Schottky contact is effectively protected from a high electric field peak. The combination of Schottky contacts with p-doped emitter regions 7 will therefore reduce leakage currents and allow to reach much higher breakdown voltages compared to pure Schottky barrier diodes such as the power semiconductor rectifier shown in FIG. 1.
In the above described power semiconductor rectifiers, the energy difference between the work function of the metal used for the first metal contact layer 5 and the conduction band edge of the drift layer 2 defines the Schottky barrier height, which defines the on-state voltage. This, in turn, is in principle bound by the choice of the metal. Consequently, the on-state voltage of the known power semiconductor rectifiers is limited by the type of metal, which has to fulfill process compatibility requirements. Therefore, the number of metals to be used for the first metal contact layer 5 in the above known power semiconductor rectifiers is very limited.
The on-state voltage should be as low as possible to minimize losses under forward bias conditions. A known approach to lower the onstate voltage while maintaining the blocking capability is the use of two different metals for the first metal contact layer resulting in a dual Schottky barrier height (SBH) rectifier. Such dual SBH rectifier is described in U.S. Pat. No. 6,362,495,B1, for example. However, using two different metals for the first metal contact layer requires additional process steps during manufacturing of the device, which involves higher costs.